Preheat the oven to 400°F (200°C). Brown the 1.5 lbs ground lamb in a large oven-safe skillet over medium-high heat for 5 to 6 minutes, breaking it up as it cooks. Spoon off all but 1 tbsp of the rendered fat.

Reduce heat to medium. Add the diced onion, 2 carrots, and 2 celery stalks and cook for 6 to 7 minutes until softened. Stir in the 4 minced garlic cloves and 2 tbsp tomato paste and cook for 2 more minutes.

Pour in the 1 cup red wine and scrape up the browned bits. Add the 1 cup beef broth, 1 tbsp Worcestershire sauce, 1 tsp thyme, and 1 tsp rosemary. Simmer uncovered for 15 minutes until the sauce thickens. Stir in the 1 cup frozen peas, taste, and season with salt and black pepper.

While the filling simmers, boil the 2.5 lbs cubed potatoes in salted water for 15 to 18 minutes until fork-tender. Drain, steam dry for 1 minute, then mash with the 4 tbsp butter, 0.5 cup warmed milk, and 0.5 cup sour cream. Season with salt and white pepper.

Spread the mashed potatoes over the filling and drag a fork across the surface to create ridges. Bake at 400°F (200°C) for 20 to 25 minutes until the ridges are golden and the filling is bubbling at the edges.

Rest for 5 minutes, scatter fresh thyme or chopped parsley over the top, and serve straight from the skillet.